The World Health Organization declared that a mysterious strain of atypical pneumonia affecting hundreds of people in Asia is a new disease. The number of people suffering from the illness in that city has almost doubled overnight.

A mysterious pneumonia has spread among hundreds of people in China and dozens in Hong Kong and Hanoi. The World Health Organization has declared it a "new communicable disease."

Peter Cordingley, a WHO information officer in Manila, said the disease is not believed to be airborne. But the illness poses a serious threat to health workers and other people caring for symptomatic patients.

"While there is evidence of infections going beyond people very close to the people who have fallen ill, the vast majority of people are medical workers. I believe that this infection can be passed only by direct and sustained contact with somebody who is infected," Mr. Cordingley said.

WHO officials also claim that the illness, which has killed at least seven people in Asia, is treatable. Some Hong Kong patients given a combination of anti-viral drugs, steroids, and respiratory support have shown improvement.

Doctors say a virus, not bacteria, is the likely cause of the illness. At least 400 people in Asia have symptoms resembling the disease, but experts have not yet determined if the outbreaks in different countries are linked.

The disease, which WHO officials have named "Severe Acute Respiratory Syndrome," starts with flu-like symptoms. It is characterized by a rapid deterioration in patients' respiratory systems.

The World Health Organization released a rare warning to airlines Sunday; saying that sick passengers might spread the disease on planes. A number of Asian governments have recommended that their citizens avoid non-essential travel to Hong Kong, Hanoi, and China.

Aside from WHO scientists, Japanese and American experts are helping governments in Hong Kong, China, and Vietnam trace the origin of the illness.

Hong Kong's health secretary Dr. Yeoh Eng-kiong says scientists have found "the index" or original patient in the Hong Kong outbreak. "It is really through a lot of detective work that we have now gradually traced back [the] most likely individual that was the source of the infection. But this is still based on the circumstantial and epidemiological evidence. That patient came in with a fever [and] did not present with pneumonia," he said.

Of the seven deaths related to atypical pneumonia in Asia, five were in mainland China last month. One patient believed to have infected at least 40 hospital staff members in Hanoi died upon returning to Hong Kong. One Hanoi hospital worker also died during the weekend.